Thanks very much for the fast replies.
The MPU has the battery modded off board (3xAA batteries) other than messy solder work I can't detect any corrosion on the board. However the power is another matter, reading through you comments I went to researching the rectifier in more details and it is now abundantly clear that it has been hacked considerably and I think bypassed in a fashion. Basically there are addition blue and brown wires soldered to the back of the rectifier directly sending power to under the play board and the solenoid board. Half the wired from J3 have been cut short just above the plug.
I've taken a couple of photo's which will explain the issue much better than I can
https://goo.gl/photos/ivKDF4ZtqceS75GX9 I've also taken a close up of CR1-CR4. In hind sight had I known more when I first posted I probably should have asked should I be replacing the rectifier board?
I'm unable to check the fuse with J3 pulled until tomorrow (is it still a worthwhile test given the current state of the wiring?), I suppose I could do some test point checking to see where power is going to from the back of the rectifier board.
